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ens Page 2 of 11 <br /> that filters the sun' s ultraviolet (UV) radiation . A <br /> depleted ozone layer allows more radiation to <br /> reach the Earth' s surface , exposing people to skin <br /> cancer, cataracts , and weakened immune systems . <br /> Increased UV can lead to reduced crop yields and <br /> disruptions in the marine food chain . <br /> While a hydrogen fuel cell economy would almost <br /> certainly improve urban air quality, the researchers <br /> found, it has the potential for unexpected <br /> consequences due to the " inevitable " leakage of <br /> hydrogen from cars , hydrogen production plants , <br /> and the transportation of the fuel . <br /> If hydrogen were to replace all fossil fuels for <br /> transportation and to power buildings , the CalTech <br /> researchers estimate that 60 to 120 trillion grams <br /> of hydrogen would be released each year into the <br /> atmosphere , four to eight times as much hydrogen <br /> as is released today by human sources . The <br /> scientists assumed a 10 to 20 percent loss rate due <br /> to leakage . <br /> The researchers say that would create a doubling <br /> or tripling of hydrogen input to the atmosphere <br /> from all sources . <br /> Because molecular hydrogen freely moves up and <br /> mixes with stratospheric air, the result would be <br /> the creation of additional water at high altitudes <br /> and, consequently, an increased dampening of the <br /> stratosphere , and, indirectly, destruction of ozone . <br /> Uncertainty remains about the effects on the <br /> atmosphere because scientists still have a limited <br /> understanding of the hydrogen cycle . At present, it <br /> seems likely such emissions could accumulate in <br /> the air . <br /> In this respect, hydrogen leakage would parallel <br /> that of chlorofluorocarbons (CFCs) - once the <br /> standard substances used for air conditioning and <br /> refrigeration . They were intended to be contained <br /> within their devices but in practice CFCs leaked <br /> into the atmosphere and attacked the stratospheric <br /> ozone layer, causing the annual holes over the <br /> South and North poles . <br /> " We have an unprecedented opportunity this time <br /> httn _ //ens -news . com/ens/lun2003 /2003 - 06 - 13 - 09 . asp 06/ 16/2003 <br />